The CRT was once the mainstream in display apparatuses of the related art. However the CRT has since been replaced by flat panel display apparatuses such as liquid crystal display apparatuses and plasma display apparatuses that are now practical and in increasing demand. Moreover, besides these apparatuses, advances are also being made in developing display apparatuses that utilize organic electro-luminescence (hereafter called organic light-emitting diode (OLED) devices), and field emission display apparatuses (FED display apparatuses) where electron sources utilizing field emissions are arrayed in a matrix and form an image from the light emitted by a fluorescent element at the anode.
Prominent characteristics of organic EL display apparatuses include the following: (1) unlike liquid crystal devices, organic EL display apparatuses emit their own light and so do not require a backlight to operate; (2) the voltage needed to emit light is less than 10 volts so power consumption can be kept low; (3) organic EL display apparatuses do not need the vacuum structure required in FED and plasma displays and so are therefore thin and lightweight; (4) organic EL display apparatuses have a short response time in the microsecond range as well as excellent motion image characteristics; and (5) their field of vision angle is a wide 170 degrees or more.
Organic EL apparatuses using thin film transistor (TFT) switching elements possess excellent image quality including contract but there are irregularities in the display characteristics on the gray scale due to irregularities or variations in individual TFT. One countermeasure in the related art for this problem is the technology shown in the examples in FIG. 9 through FIG. 23.
FIG. 19 is a schematic of a drive circuit for the pixel section in the first example of the related art. In FIG. 19, an OLED drive TFT3 from the power supply line 51, a lighting TFT switch 2, an organic EL light-emitting element (OLED element 1) are connected in series, and one end of the OLED element 1 is connected to reference (voltage) potential. Here, the reference potential is the voltage potential used as a reference for the display apparatus, and is broad term that includes ground potential. The drive circuit controls the OLED element 1 brightness to form the image by regulating the electrical current flowing in the OLED element 1. The lighting TFT switch 2 controls the flow or non-flow of current in the OLED element 1.
The OLED drive TFT3 controls light emission gray scale intensity from OLED element 1 according to the signal from the signal line 54. In other words, a storage capacitor 4 connected to the gate of the OLED drive TFT3 accumulates the signal from the signal line 54 and the gray scale is displayed by controlling the current flowing in OLED drive TFT3 according to the voltage potential in the storage capacitor 4. However, there are large variations in the threshold voltage Vth due to individual manufacturing irregularities in the OLED drive TFT3. In order to compensate for these threshold Vth irregularities, a reset TFT 5 switch is turned on, and current is made to flow for a short period in the OLED drive TFT 3. Turning the switch on and making current flow sets the gate voltage V10 on OLED drive TFT 3 to a value also including the threshold voltage Vth of OLED drive TFT3 so the OLED element emits light to faithfully reproduce the image signal.
FIG. 20 is a timing chart for driving the drive circuit in FIG. 19. As shown in the upper section of FIG. 2, this drive circuit is separated into a write period in the first half of a single frame and a light emission time in the latter half. The drive circuit writes the gray scale signal in each pixel during the write period. FIG. 20 shows the state where data is written at the write operation position in the scanning line sequence. The lower part of FIG. 20 shows the timing for writing on the pixels. In FIG. 20, the drive circuit first turns the reset TFT switch 5 on and then shorts V10 and V12 shown in FIG. 19. Current next flows in the OLED drive TFT3 when the lighting TFT switch 2 turns on. The time where the lighting TFT switch 2 and the reset TFT switch 5 simultaneously turn on is tc4 as shown in FIG. 20. If tc4 is sufficiently long, then the gate voltage V10 for OLED drive TFT3 converges on the values for the intersection point of the straight line of V10=V12 with the characteristic curve for drain voltage 12 and gate voltage V10 of LED drive TFT3. This state is shown in FIG. 21 through FIG. 23. The vertical axis in FIG. 21A shows the V10 value in FIG. 19, and the horizontal value is the time. The V12 values for the point in time that lighting TFT switch 2 turned on is undefined here since it depends on the display status of the prior frame. This V12 value in other words is seen as between a voltage of ground potential or higher to ground potential. The reset TFT switch 5 is turned on at this time so V12 and V10 are the same value. If tc4 is sufficiently long here, then as described above, the gate voltage 10 for OLED drive TFT3 converges on the values for the intersection point of the straight line of V10=V12 with the characteristic curve for drain voltage 12 and gate voltage V10 of LED drive TFT3, or in other words converges on Vres10. This operation is the same in FIG. 21B and FIG. 21C. The voltage threshold Vth for OLED drive TFT3 may differ among FIG. 21A, FIG. 21B and FIG. 21C.
FIG. 22 shows how the voltage potential for V10 in FIG. 19 is set for OLED drive TFT3 possessing different characteristics. In FIG. 19, when the lighting TFT switch 2 is on, then the OLED drive TFT3 and OLED element 1 may together form an inverter. The curve in FIG. 22 shows characteristics of the drain voltage V12 and gate voltage V10 of OLED drive TFT3, and the straight line in FIG. 22 shows the V10 equals V12. The reset TFT switch 5 here shorts the gate and drain of OLED drive TFT3, which causes the gate of OLED drive TFT3 to be set to a voltage decided by the intersection of the straight line of V10=V12 with the characteristic curve for OLED drive TFT3. Characteristic curves for the three OLED drive TFT3 of different threshold voltages Vth are drawn in FIG. 22. As shown in FIG. 22, the gate voltage for OLED drive TFT3 is set to a threshold voltage Vth that includes the different threshold voltage Vth of each OLED drive TFT.
The operation point Vres10 for the characteristic MAX in FIG. 22 corresponds to the Vres10 in FIG. 21A. The operation point Vres11 for characteristic TYP in FIG. 22 corresponds to Vres11 in FIG. 21B. The operation point Vres12 for the characteristic MIN in FIG. 22 corresponds to Vres12 in FIG. 21A. These operation points are reflected in the Vth of OLED drive TFT13. Based on these operation points, the image signal from line 54 is written in the storage capacitor 4. FIG. 23 shows the relation between the value V12 roughly equal to the positive voltage for the OLED element and signal voltage V11 shown in FIG. 19. As shown in FIG. 23, even if there are variations in the OLED drive TFT there is almost no effect on the signal voltage V11 and drive voltage for the OLED element or in other words, the light emission characteristics.
The second example of the related art for compensating for variations in the gray scale display is described in FIG. 27 through FIG. 34. FIG. 27 is a drive circuit for driving one pixel. In FIG. 27, the OLED drive TFT3, the lighting TFT switch 2 and the OLED element 1 are connected in series from the power supply line 51. The lighting TFT switch 2 controls emission (or no emission) in the OLED element 1. The OLED drive TFT 3 shows the gray scale display at the established voltage based on the charge accumulated in the first storage capacitor 41. In this case also, the reset TFT switch 5 is utilized to suppress variation in the light emission characteristics of OLED element 1 due to variations in the (threshold voltage) Vth of the OLED drive TFT3.
FIG. 28 is a drawing for describing the operation of the drive circuit in FIG. 27. The TFT used in FIG. 27 is the P-type so the TFT turns on when a negative signal arrives. In the comparative example 2, when a gray scale voltage is written in each pixel in a signal frame period, that gray scale voltage is maintained to make the OLED element 1 emit light. In the initial state in FIG. 28, the lighting TFT switch 2 is on. This state turns on the select switch 6. Data from the signal line 54 is in this way inputted to the pixels. Next, when the reset TFT switch 5 turns on, the drain voltage V15 for OLED drive TFT3, and the gate voltage V13 for OLED drive TFT3 are shorted together. The lighting TFT switch 2 next turns off and the V13 voltage shown in FIG. 27 converges on a lower voltage value of Vth for OLED drive TFT3 than the power supply voltage. The reset TFT switch 5 then turns off and when the signal voltage from signal line 54 is written, charges reflecting the signal voltage are accumulated in the second storage capacitor 42 and the first storage capacitor 41 regardless of the Vth of LED drive voltage TFT3, and an accurate gray scale can be displayed.
However, the initial value of the V13 voltage potential shown in FIG. 27 is dependent on the display state of the previous frame so the value is undefined. Namely, this value may appear as at voltage between ground potential and the supply voltage or higher. After the reset switch is turned on, the above operation converges V13 at a value where the Vth of the OLED drive TFT3 is subtracted from the supply voltage, in the period of time tc5 until the lighting switch TFT 2 turns off. This state is shown in FIGS. 29A to 29C. As shown in FIG. 29A, the value for V13 in FIG. 27 converges on Vres13 when the Vth is small. In FIG. 29B the value for V13 converges on Vres14, when the Vth is at reference potential. In FIG. 29C, the V13 converges on Vres15 when the Vth is large.
FIG. 30 shows the OLED drive TFT3 input/output characteristics. In FIG. 30 the vertical axis shows the drain voltage V15 for OLED drive TFT3 forming the anode for OLED element 1 and the horizontal axis shows the gate voltage V13 for OLED drive TFT3 during pixel lighting. If there are variations among the OLED drive TFT3 characteristics, then the gate voltage for OLED drive TFT3 is converged respectively on the Vres13, Vres14, and Vres15 according to the OLED drive TFT3 characteristics. The signal voltage overlaps these converged voltages so light emission on the gray scale for OLED element 1 accurately reflects this signal voltage. This state is shown in FIG. 31. In FIG. 31, the vertical axis shows the drain voltage V15 for the OLED drive TFT3 forming the anode of the lighting OLED element 1, while the horizontal axis is the signal input voltage V14. As shown in FIG. 31, the variations in light emission intensity in OLED element 1 are small even if there are variations in the threshold voltage Vth of OLED drive TFT3.
The related art described above are disclosed in “JP-A No. 2003-5709”, “JP-A No. 2003-122301”, and “Digest of Technical Papers, SID98, pp. 11-14”.